Metal halide discharge lamps typically have a discharge vessel of quartz glass. Recently, however, attempts have been made to improve the color rendition of these lamps. The higher operating temperature that this requires can be achieved with a ceramic discharge vessel. Typical output levels are from 100 to 250 W. The ends of the tubular discharge vessel are typically closed with cylindrical ceramic end plugs, into the middle of which a metallic power lead-through is inserted.
A similar technique is employed with high-pressure sodium vapor lamps. Both tubular and pronglike versions made of niobium are known (British Patent 1 465 212 Rigden and U.S. Pat. No. 4,376,905, Kerekes which are fused into a ceramic end plug by means of glass solder or melt ceramic. Direct, glass-solder-free sintering for nioblum/tubes has also been described (U.S. Pat. No. 4,545,799, Rhodes et. al. The special feature of high-pressure sodium vapor discharge lamps is that the filling includes sodium amalgam, which is often contained in a reservoir in the interior of a niobium tube used as a lead-through. An especially simple possibility for filling and evacuating the discharge vessel is for one of the two niobium tubes to have a small opening in the vicinity of the electrode shaft mounted on the tube, in the interior of the discharge vessel, so that evacuation and filling with the amalgam and inert gas can be done through this opening (U.S. Pat. No. 4,342,938, Strok). After the filling process is concluded, the outside protruding end of the niobium tube is closed in gas-tight fashion by pinching, followed by welding. Nevertheless, the opening in the vicinity of the electrode shaft always remains open, so that during operation communication between the interior of the discharge vessel and the interior of the lead-through tube, acting as a cold spot, will be assured.
A different closure technique for high-pressure sodium vapor lamps is known from U.S Pat. No. 4,011,480, Jacobs et. al. It uses tubular lead-throughs of tungsten, molybdenum or rhenium, which are fused in gas-tight fashion into the plugs with the aid of a ceramic cylindrical shaped part in the interior of the tube by means of melt ceramic. Pinching the outer tube/end after the filling process is concluded must then be omitted, because these metals, in contrast to niobium, are known to be very brittle and can therefore be machined only with difficulty. The closure techniques that are known for niobium tubes cannot therefore be readily adopted. Instead, the ceramic shaped part is equipped with an axial bore, which during evacuation and filling cooperates with an opening in the tube in the vicinity of the electrode shaft. After the filling, the axial bore of the shaped part is closed with melt ceramic, making machining of the brittle molybdenum-like metal unnecessary. However, this technique is very inconvenient and therefore expensive and time-consuming.